Proposals are made, in particular in documents FR-2 930 759 and FR-2 930 760, to place motors in association with certain undercarriages so as to enable aircraft to taxi under their own power. Such a motor may be constituted, for example by a microturbine installed in an undercarriage. Nevertheless, such devices are complex and require fuel or compressed air to be delivered to the undercarriage, thereby adding pipework and thus likely to give rise to additional weight and manufacturing costs for the aircraft.
Proposals have also been made to provide an undercarriage with an electric motor powered by the aircraft's auxiliary power unit (APU).
However, the power needed to move an aircraft on the ground is generally of the same order of magnitude as the electrical power available from the APU for powering the other systems that use power on the ground. This means that the dimensions and the capacity of the APU need to be increased very significantly, as does its weight. An important part of the saving in fuel consumption is then lost as a result of the additional weight.
Furthermore, conveying that amount of electrical power to the undercarriage requires a dedicated power line to be installed, thereby contributing to making the design of the aircraft more complex and increasing its weight.
Another drawback of that solution relates to delivering hydraulic or electrical power to the systems needed for an aircraft that is taxiing and preparing for flight. Nowadays, the APU is not designed to be capable nominally, of powering all of the needs of the airplane. One method that has been envisaged for enabling it to power the motor consists in connecting it to the overall power supply network of the aircraft. The APU can thus deliver the power needed by each of the power-consuming devices while the airplane is taxiing. However that also leads to the aircraft being made significantly more complex and heavier.
It is also possible to envisage defining two independent stages of taxiing. A first stage makes use solely of the motors powered by the APU and forms a stage merely of transferring the aircraft from the terminal to the vicinity of the takeoff runway. This stage may be performed after minimum verification of the preparation of the aircraft and under comfort conditions for the passengers that are potentially degraded. The second stage, shortly before takeoff, is performed solely with the engines. It serves to prepare the aircraft and in particular to perform the usual verifications (“check lists”) and to put the passengers into conditions of comfort and safety that are appropriate for flying, e.g. so as to be able to read the safety instructions. However that method raises operational difficulties that are difficult for an airline to accept. In particular, in the event of an anomaly being detected shortly before takeoff, the aircraft needs to return to the terminal in order to resolve the problem.